The present invention relates generally to rotatable members that are able to achieve balanced conditions throughout a range of rotational speeds. The present invention also relates to methods and systems for dynamically balancing rotatable members through the continual determination of out-of-balance forces and motion to thereby take corresponding counter balancing action. The present invention additionally relates to methods and systems for actively placing inertial masses within a rotating body in order to cancel rotational imbalances associated with the rotating body thereon. In addition, the present invention relates to algorithms that achieve the desired point-mass and point-location counterbalance actions through a distributed placement of mass across a limited number of receiving receptacles on the rotating body.
When rotatable objects are not in perfect balance, nonsymmetrical mass distribution creates out-of-balance forces because of the centrifugal forces that result from the rotation of the object. This mass unbalance can result in machine vibrations that are synchronous with the rotational speed. Such vibrations can lead to excessive wear and unacceptable levels of noise.
Balancing of a rotatable body is commonly achieved by adjusting a distribution of moveable, inertial masses attached to the body. In general, this state of balance may remain until there is a disturbance to the system. A tire, for instance, can be balanced once by applying weights to it and the tire will remain balanced until it hits a very big bump or the weights are removed. However, certain types of bodies that have been balanced in this manner will generally remain in balance only for a limited range of rotational velocities. One such body is a centrifuge for fluid extraction, which can change the degree of balance as speed is increased and more fluid is extracted.
Many machines are also configured as freestanding spring mass systems in which different components thereof pass through resonance ranges during which the machine may become out of balance. Additionally, such machines may include a rotating body loosely coupled to the end of a flexible shaft rather than fixed to the shaft as in the case of a tire. Thus, moments about a bearing shaft may also be created merely by the weight of the shaft. A flexible shaft rotating at speeds above half of its first critical speed can generally assume significant deformations, which add to the imbalance.
Machines of this kind usually operate above their first critical speed. As a consequence, machines that are initially balanced at relatively low speeds may tend to vibrate excessively as they approach full operating speed. Additionally, if one balances to an acceptable level rather than to a perfect condition (which is difficult to measure), the small remaining xe2x80x9cout-of-balancexe2x80x9d will progressively apply greater force as the speed increases. This increase in force is due to the fact that F is proportional to rxcfx892, (note that F is the out-of-balance force, r is the radius of the rotating body and xcfx89 is its rotational speed).
The mass unbalance distributed along the length of a rotating body gives rise to a rotating force vector at each of the bearings that support the body. In general, the force vectors at respective bearings are not in phase. At each bearing, the rotating force vector can be opposed by a rotating reaction force, which can be transmitted to the bearing supports as noise and vibration. The purpose of active, dynamic balancing is to shift an inertial mass to the appropriate radial eccentricity and angular position for canceling the net unbalance. At the appropriate radial and angular distribution, the inertial mass can generate a rotating centrifugal force vector equal in magnitude and phase to the reaction force referred to above. Although rotatable objects find use in many different applications, one particular application is a rotating drum of a washing machine.
Many different types of balancing schemes are known to those skilled in the art. U.S. Pat. No. 5,561,993, which was issued to Elgersma et al. on Oct. 22, 1996, and is incorporated herein by reference, discloses a self-balancing rotatable apparatus. Elgersma et al. disclosed a method and system for measuring forces and motion via accelerations at various locations in a system. The forces and moments were balanced through the use of a matrix manipulation technique for determining appropriate counterbalance forces located at two axial positions of the rotatable member. The method and system described in Elgersma et al. accounted for possible accelerations of a machine, such as a washing machine, which could not otherwise be accomplished if the motion of the machine were not measured. Such a method and system was operable in association with machines not rigidly attached to immovable objects, such as concrete floors. The algorithm disclosed by Elgersma et al. permitted counterbalance forces to be calculated even when the rotating system (such as a washing machine), was located on a flexible or mobile floor structure combined with carpet and padding between the washing machine and a rigid support structure.
U.S. Pat. No. 5,561,993 thus described a dynamic balance control algorithm for balancing a centrifuge for fluid extraction. To accomplish such balance control, balance control actions may place mass at the periphery of axial control planes on the centrifuge. Related sensor responses to balancing control actions on a centrifuge can be modeled and utilized to determine control actions (i.e., balance control actions) that would serve to drive an associated system toward a balanced state. Such a system is generally time variant, such that the control models utilized therein may need to be routinely updated based on the measured response to a previous control action, which is a variation of perturbation theory, well known in the art. The control actions may require multiple control actuators, generally one per axial control plane, although multiple actuators at multiple control planes may emulate additional virtual control planes.
The determined counterbalance control action (i.e., balance control action) is generally represented as a force or mass (regarding rxcfx892) magnitude to be applied at a specific angular point along the periphery of an axial control plane on the centrifuge. A variety of control action actuation techniques have been developed that generally depend on placement of mass across a ringed distribution of retaining receptacles on the rotating apparatus, to affect the desired control action. For a large number of retaining receptacles, counterbalance mass could simply be distributed at an angular span symmetric about the determined counterbalance angular point. Such a technique for distributing counterbalance mass does not take into account that a mechanically and monetarily feasible device would be confined to having a limited number of receptacles. With fewer receptacles the mass is less confined and it is not possible to affect the desired control action with a simple symmetric distribution about the determined counterbalance angular point.
Based on the foregoing, it can be appreciated that a method and system are required to affect a desired control action whereby counterbalance mass is distributed about the rotating system utilizing a limited number of retaining receptacles. The invention disclosed herein thus addresses these needs and the related concerns.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is one aspect of the present invention to provide methods and systems in which rotatable members can achieve balanced conditions throughout a range of rotational speeds.
It is another aspect of the present invention to provide methods and systems for dynamically balancing rotatable members through the continual determination of out-of-balance forces and motion to thereby take corresponding counter balancing action.
It is yet another aspect of the present invention to provide methods and systems for dynamically balancing rotatable members wherein counterbalancing action distributes mass in receptacles occupying a given angular span about the rotating system, wherein the receptacles are greatly limited in number and mass additions are confined to these receptacles.
It is still another aspect of the present invention to provide methods and systems for dynamically balancing rotatable members where the amount of counter balancing mass placed at various sites about the rotating system is determined through a mass placement algorithm.
In accordance with various aspects of the present invention, methods and systems are disclosed herein for distributing counterbalance mass across a limited-number collection of retaining receptacles in order to affect a control action to dynamically balance the rotating system. A determined counterbalance control action can be represented as a force or mass (regarding rxcfx892) magnitude to be applied at a specific angular point along the periphery of an axial control plane on a rotating apparatus. The control action can be accomplished through the placement of mass across a ringed distribution of retaining receptacles on the rotating apparatus. With the number of receptacles greatly restricted, a mass placement algorithm may determine how mass can be proportioned across a span of receptacles and incrementally applied to efficiently affect the desired radial counterbalance force at its designated angular point-location. The counterbalance mass distribution can be determined such that force elements normal to the counterbalance angular point-location, and introduced because of the distributed control action, cancel each other or sum to zero.
Depending on the limited number of retention receptacles and the desired angular span for distributing counterbalance mass, the mass placement algorithm may operate by placing as much mass as possible starting with a primary receptacle or cup associated with the counterbalance angular point-location. It then may sequence through alternating, adjacent receptacles, until the desired angular span is reached, where the last cup is used to zero any remaining force component that is normal to the desired counter-balance action direction. There may be additional limit and sizing activities that ensure that an integer number of steps is used to achieve the desired counterbalance point-mass effect and that actuator timing limits are not violated. As the number of retention receptacles increases, the need for the mass placement algorithm becomes less critical as the error in distributed mass placement is bound by one receptacle.